Fruit fly decimates area olive crops

A recent increase in fruit flies has area olive growers seeing pits. Literally.

Glen Ellen resident Joshua Rymer has a small crop of olive trees on his property, which he grows and harvests as a hobby. But this year, there won’t be any olives to harvest.

The olive fruit fly, which has been in the Valley for more than 10 years, decimated Rymer’s olive crop.

The fly, Rymer said, lays eggs in the young olive in the summer period and those eggs and pupae eat the olives, destroying the flesh from the inside out.

“We looked around in October when the olives were ripening, then in the beginning of November, and you can see how deformed they’ve become,” Rymer said, adding he will have to pick the entire crop and dispose of it. “If you leave it (an infested tree) in place, it becomes home for next year’s fly infestation.”

Effects of the fruit fly were also noticed at McEvoy Ranch, which grows 80 acres of olives in Petaluma and is lauded as one of the nation’s largest organic olive oil producers. Ranch manager Samantha Dorsey said the ranch lost 2 percent of its crop to fruit flies, and while this was not a huge devastation to the commercial growing operation, she did notice a spike in the fly’s population this year.

“We had to spray much more frequently to maintain that pristine fruit,” Dorsey said, noting this is the worst year she’s seen for the pest.

Dorsey said early and frequent monitoring of olive trees is key to noticing population spikes and will give appropriate indicators for when and how often to spray.

It’s been such a horrendous year for the olive fruit fly,” Dorsey said. “Many of our milling customers who usually bring their fruit in to mill had to cancel because they lost their entire crop.”

Rymer, whose crop yields only about 200 to 300 pounds of olives, said that while he did put out traps and spray, he may have not sprayed as frequently as commercial growers because it is not as economical.

The mild summer that contributed to the early harvest may be what helped the olive fruit fly survive, according to Dorsey, who said when the temperatures get hot (upward of 90 degrees) mortality rates in the fruit flies and their pupae are increased.

One of the most important precautions, Dorsey said, is for homeowners who have olive trees to either tend to their trees or get rid of them. “So many untended trees in the Bay Area and California are harboring fruit fly populations and contributing to population increases and infestations of neighboring crops.

Rymer said many of his fellow hobbyist growers had their crops wiped out by the fruit fly as well. He and Dorsey both agree that because of the infestation, there will be less olive oil produced this year.

Dozens of olive growers were turned away from the mill at McEvoy Ranch because their olives were too infected to press, Dorsey said.

Because so many homeowners’ crops were devastated by the fruit fly, Dorsey said she talked with many who were worried the fruit flies may have become resistant to the spray compound used, GF-120, or spinosad.

But, Dorsey said, she doesn’t believe the flies are resistant because the ranch only experienced minimal damage. The key, she said, is for olive growers to be on top of their crop maintenance program and work on the timing of their spraying and their application technique.

Paul Vossen, who has been a University of California Cooperative Extension farm advisor in Sonoma and Marin Counties since 1980, explained that spinosad is an organic pesticide that comes from the bacteria Saccharopolyspora spinosa, and makes the flies die when consumed.Vossen notes resistance usually comes from a mutation. “You would see that in maybe one or two locations, but this has happened all over California, all at the same time. It’s really a population issue.”

Vossen said, what it comes down to is an increase in population of the fly that growers weren’t prepared for. The fly, which he said probably comes from somewhere in Africa and only eats olives, reproduces and builds up its population very quickly. “We’ve had years in a row where the population was low, so producers got complacent and (with the population increase) this year it kind of snuck up on them.”

The pest problem, Vossen said, is not just limited to Sonoma County, but has affected the whole state. The fly was introduced to California a decade ago and has caused considerable damage this year.

Vossen said the fly lands on the flesh of the olive and bores holes into it to lay its eggs inside. “You can have a certain little amount of damage, even if it (the fruit) gets a little tunneling, but where you run into a problem is when the larvae starts to get real big and a fungus or something is introduced to the weakened olive, then the fruit rots.”

Dorsey said it is crucial to begin monitoring the crops in the spring when the temperature gets warm, and to start putting traps out for the flies. For example, she explained, she puts 10 traps out on the ranch’s 80 acres and checks the traps every week. When she notices a spike in the population, she sprays.

Dorsey said while the ranch did mill more infected olives than it should have this year, it’s important for people to eat that oil within the first two weeks or make it into soap to get a sellable product. “The fresh oil won’t last because of all of the rotten bug flesh in there,” she said.

In the spring, Dorsey plans to teach a workshop in which she will discuss olive crop management and pest control. Information about the workshop will be posted online at mcevoyranch.com when dates and other details are determined.

For more information on the olive fruit fly and crop management, go to Vossen’s website at cesonoma.ucdavis.edu